It is known to design a state estimator employing a non-linear Kalman filter that estimates an indicated torque of an engine from an observed value of a crank angle obtained from a crank sensor, using a mathematical model.
Japanese Laid-open Patent Publication No. 2009-275618, Japanese Laid-open Patent Publication No. 2006-112373, Japanese Laid-open Patent Publication No. 2012-188930, and Yuzuru Itoh, Kouji Higashi, and Masami Iwase: “UKF-based Estimation of Indicated Torque for IC engines Utilizing Nonlinear Two-inertia Model,” Proc. of 51st IEEE Conference on Decision and Control, December (CDC), pp. 4077-4082, (2012) discuss related art.